In the previous project, an adjustable power supply circuit having an output voltage in the range between 0V and 30V with the maximum current capacity of 2A was designed. A lot of times required DC supply should have both positive and negative voltages. In the previous project, the negative voltage could be provided to a device only by reversing the terminal connections manually. The circuit will input 220V-230V AC and generates a variable DC voltage in the range of +/- 1.25 V to +/-22V at the output. This power supply can provide a maximum current of 1A at the output. For making an adjustable power supply that would have both negative and positive voltages, a center tape transformer needs to be employed in the circuit design.
In this project, an adjustable regulated symmetrical positive and negative power supply is designed. For reducing any fluctuation and ripples at the output the supply needs to be regulated so that it can provide a constant voltage at the output. Again like the previous project, the voltage is adjusted using a variable resistor. This power supply provides regulated as well as the adjustable voltage at the output.
The design of a power supply circuit is a step by step process involving stepping down AC voltage, converting AC voltage to DC voltage, Smoothing DC voltage, Compensating transient currents, Voltage regulation, Voltage variation and Current amplification and Short circuit protection.
Components Required –
Fig. 1: List of Components required for Adjustable +/- 1.25V to +/-22V 1A Power Supply
Block Diagram –
Fig. 2: Block Diagram of Adjustable +/- 1.25V to +/-22V 1A Power Supply
Circuit Connections –
The circuit is assembled in stages with each stage serving a specific purpose. For stepping down the 230 V AC, a 24 V – 0 – 24 V transformer is taken. The secondary coil of the transformer is connected with a full-bridge rectifier. The full bridge rectifier is built by connecting four 1N4007 diodes to each other designated as D1, D2, D3 and D4 in the schematics. The cathode of D1 and anode of D2 is connected to one of the secondary coil and cathode of D4 and anode of D3 is connected to other end of the secondary coil. The cathodes of D2 and D3 are connected from which one terminal is taken out from the output of rectifier and anodes of D1 and D4 are connected from which other terminal is taken out from the output from full-wave rectifier. A wire is drawn from the center tape of the transformer which serves as the ground for the positive and negative DC outputs.
The capacitors of 100 uF (shown as C1 and C2 in schematics) is connected between the output terminals of full-wave rectifier and center tape of the transformer for smoothing purpose. For voltage regulation, LM317T and LM337 are connected in parallel to the smoothing capacitor. The variable resistances are connected in series to the voltage regulator ICs for voltage adjustment and capacitors of 10 uF (shown as C5 and C6 in schematics) are connected in parallel at the output for compensating transient currents. There are two diodes connected between the Input voltage and output voltage terminals of the voltage regulator ICs for short circuit protection.
Get the schematic diagram drawn or printed on a paper and make each connection carefully. Only after checking each connection made correctly, plug in the power circuit to an AC supply.
How the project works –
The power circuit operates in well-defined stages with each stage serving a specific purpose. The circuit operates in the following stages –
1. AC to AC Conversion
2. AC to DC Conversion – Full Wave Rectification
4. Compensation of Transient Current
5. Voltage Regulation
6. Voltage Adjustment
7. Short Circuit Protection
AC to AC conversion
The voltage of Main Supplies (Electricity fed by the intermediate transformer after stepping down line voltage from generating station) is approximately 220-230V AC which further needs to be stepped down to 24V level. To reduce the 220V AC to 24V AC, a step-down transformer with center taping is used. The use center tap transformer is to generate both positive and negative voltage at the input. The center tape will provide ground to the circuit and the remaining two terminals will provide positive and negative voltage.
The circuit takes some drop in the output voltage due to resistive loss. Therefore a transformer of high voltage rating greater than the required 22 V needs to be taken. The transformer should provide 1A current at the output. The most suitable step-down transformer that meets the mentioned voltage and current requirements is 24V-0-24V/2A. This transformer step downs the main line voltage to +/-24V AC, as shown in the below image.
Fig. 3: Circuit Diagram of 24-0-24V Transformer
AC to DC conversion – Full Wave Rectification
The stepped down AC voltage needs to be converted to DC voltage through rectification. The rectification is the process of converting AC voltage to DC voltage. There are two ways to convert an AC signal to the DC one. One is half wave rectification and another is full wave rectification. In this circuit, a full-wave bridge rectifier is used for converting the 48V AC to 48V DC. The full wave rectification is more efficient than half wave rectification since it provides complete use of both the negative and positive sides of AC signal. In full wave bridge rectifier configuration, four diodes are connected in such a way that current flows through them in only one direction resulting in a DC signal at the output. During full wave rectification, at a time two diodes become forward biased and another two diodes get reverse biased.
Fig. 4: Circuit Diagram od Full Wave Rectifier
During the positive half cycle of the supply, diodes D2 and D4 conduct in series while diodes D1 and D3 are reverse biased and the current flows through the output terminal passing through D2, output terminal and the D4. During the negative half cycle of the supply, diodes D1 and D3 conduct in series, but diodes D1 and D2 are reverse biased and the current flow through D3, output terminal and the D1. The direction of current both ways through the output terminal in both conditions remain the same.
Fig. 5: Circuit Diagram showing positive cycle of Full Wave Rectifier
Fig. 6: Circuit Diagram showing negative cycle of Full Wave Rectifier
The 1N4007 diodes are chosen to build the full wave rectifier because they have the maximum (average) forward current rating of 1A and in reverse biased condition, they can sustain peak inverse voltage up to 1000V. That is why 1N4007 diodes are used in this project for full wave rectification.
As the name suggests it is the process of smoothing or filtering the DC signal by using a capacitor. The output of the full-wave rectifier is not a steady DC voltage. The output of the rectifier has double the frequency of main supplies but still containing ripples. Therefore, it needs to be smoothed by connecting capacitors (shown as C1 and C2 in schematics) in parallel to the output of full wave rectifier. The capacitor charges and discharges during a cycle giving a steady DC voltage as the output. So, capacitors (shown as C1 and C2 in schematics) of high value are connected to the output of rectifier circuit. These capacitors act as filtering capacitors which bypass all the AC through them to ground. At the output, the mean DC voltage left is smoother and ripple free.
The capacitors C3 and C4 are connected with adjustment pin. These capacitors prevent ripple from being amplified as the output voltage is increased.
Fig. 7: Circuit Diagram of Smoothing Capacitor
Compensating Transient Currents
At the output terminals of the power circuit, capacitors C5, C6, C7, and C8 are connected in parallel to the output terminals. The capacitor C5 and C6 help in fast response to load transients. Whenever the output loads current changes then there is an initial shortage of current, which can be fulfilled by this output capacitor.
The capacitor C7 and C8 are ceramic capacitors, the impedance or ESR of ceramic is low as compared to an electrolytic capacitor. Therefore C7 and C8 are used in parallel to electrolytic capacitor just to decrease the equivalent output impedance.
The output current variation can be calculated by
Output current ,Iout = C (dV/dt) where
dV = Maximum allowable voltage deviation
dt = Transient response time
Considering dv = 100mV
dt = 100us
In this circuit a capacitor of 1 uF is used so,
C = 1uF
Iout = 1u (0.1/100u)
Iout = 1mA
This way it can be concluded that output capacitor will respond for 1mA current change for a transient response time of 100 us.
Fig. 8: Circuit Diagram of Transient Current Compensator
The power circuit should provide regulated and constant voltage without any fluctuation or variation. For voltage regulation, a linear regulator is needed in the circuit. The aim of using this regulator is to maintain a constant voltage of a desired level at the output. For providing a regulated 1.25V to 22V LM317 IC is used and for -1.25 to -22V at the output LM337 IC is used. Both the IC are capable of providing a current of 1.5A, so are well suited for the current requirement of 1A. In this circuit, LM317 and LM337 will provide an adjustable voltage corresponding to its input voltage. Both of these ICs are capable of load regulation. They will provide regulated and stabilized the voltage at the output irrespective of the fluctuation in the input voltage and load current.
LM317 is a positive voltage regulator which gives output in the range of 1.25V to 37V with input voltage up to 40V. Contrary to LM317, the LM337 is a negative voltage regulator which provides -1.25V to -37V with input voltage up to -40V. At the output, both can provide a maximum current of 1.5A as per the datasheet under the optimum conditions.
For setting a desired voltage at the output, resistive voltage divider circuit is used between the output pin and the ground (center tape of the transformer). The voltage divider circuit has one programming resistor (fixed resistor) and another a variable resistor. By taking a perfect ratio of feedback resistor (fixed resistor) and a variable resistor, desired value of output voltage corresponding to the input voltage can be obtained. In this circuit, the R1 and R2 resistances are used as a programming resistance for 317 and 337 respectively. The variable resistances RV1 and RV2 are used to vary the output voltage at 317 and 337 respectively.
The LM317 has the following internally tolerable power dissipation –
Pout = (Maximum operating temperature of IC)/ (Thermal Resistance, Junction−to−Ambient + Thermal Resistance, Junction−to−case)
Pout = (150) / (65+5) (values as per the datasheet)
Pout = 2W
Similarly, LM337 has the following internally tolerable power dissipation –
Pout = (Maximum operating temperature of IC)/ (Thermal Resistance, Junction−to−Ambient + Thermal Resistance, Junction−to−case)
Pout = (125) / (70+3) (values as per the datasheet)
Pout = 1.7W
Therefore, 317 and 337 internally can sustain up to 2W and 1.7W power dissipation respectively. Above 2W and 1.7W, the ICs will not tolerate the amount of heat generated and will start burning. This can cause a serious fire hazard also. So heat sinks are needed to dissipate the excessive heat from the ICs.
The output voltage can be varied by using the adjust pin of 317 and 337 ICs. The variable resistors RV1 and RV2 provide the output voltage from 1.25V to 22V and -1.25 to -22V respectively.
Short Circuit Protection
A diode D5 is connected between the voltage input and voltage output terminals of 317 IC so that it can prevent the external capacitor from discharging through the IC during an input short circuit. When the input is shorted then the cathode of the diode is at ground potential. The anode terminal of the diode is at high voltage since C5 is fully charged. Therefore in such a case, the diode is forward biased and all the discharging current from capacitor passes through the diode to the ground. This saves the LM317 IC from the back current.
In a similar way, a diode D6 is connected between the voltage input and voltage out terminals of 337 IC to prevent the IC from discharging of the capacitor C6 through the IC when the input is shorted.
Fig. 9: Circuit Diagram of Short Circuit Protection
Testing and Precautions
The following precautions should be taken while assembling the circuit:
• The current rating of the step-down transformer, bridge diodes and voltage regulator ICs must be greater than or equal to the required current at the output. Otherwise, it will be unable to supply the required current at the output.
• The voltage rating of the step-down transformer should be greater than the maximum required output voltage. This is due to the fact that, the 317 and 337 ICs take voltage drop of around 2 to 3 V. Thus input voltage must be 2V to 3V greater than the maximum output voltage and should be in the limit of the input voltages of LM317 and LM337.
• The capacitors used in the circuit must be of higher voltage rating than the input voltage. Otherwise, the capacitors will start leaking the current due to the excess voltage at their plates and will burst out.
• A capacitor should be used at the output of rectifier so that it can handle unwanted mains noise. Similarly, Use of a capacitor at the output of the regulator is recommended for handling fast transient changes and noise at the output. The value of output capacitor depends on the deviation in the voltage, current variations and transient response time of the capacitor.
• A protection diode should always be used while using a capacitor after a voltage regulator IC, for preventing the IC from back current while discharging of the capacitor.
• For driving the high load at the output, heat sink should be mounted at the holes of the regulator. This will prevent the IC from blowing off due to heat dissipation.
• As the regulator ICs can draw current up to 1A only, a fuse of 1A needs to be connected. This fuse will limit the current in the regulator up to 1A. For current above 1A, the fuse will blow off and this will cut the input supply from the circuit. This will protect the circuit and regulator ICs from current greater than 1A.
Once the circuit is assembled, it’s time to test it. Plug in the circuit to main supplies and change variable resistance. Take the voltage and current readings at the output terminal of the power circuit using a multimeter. Then connect fixed resistances as load and check the voltage and current readings again.
At the LM317 side of the circuit, input voltage was 24V and on adjusting the variable resistance, the output voltage read between 1.25 to 22V when no load was connected.
When a load is connected at the output the maximum voltage is read 20V. With a load of 50Ω resistance, the output voltage is read 16 V showing a voltage drop of 4 V. The output current is measured 300 mA so the power dissipation at load of 50Ω resistance is as follows-
Pout = (Vin – Vout)*Iout
Pout = (24-16) *(0.3)
Pout = 2.4W
At the LM337 side of the circuit, input voltage was -24V and on adjusting the variable resistance, the output voltage read between -1.25 to -22V when no load was connected.
When a load is connected at the output the maximum voltage is read -20V. With a load of 50Ω resistance, the output voltage is read 17.5 V showing a voltage drop of 2.5 V. The output current is measured 320 mA so the power dissipation at load of 50Ω resistance is as follows-
Pout = (Vin – Vout)*Iout
Pout = (-24 – (-17.5)) *(0.32) (power dissipation cannot be negative)
Pout = 2.08W
During testing of the circuit, it was analyzed that when current demand increases at the output then the output voltage starts reducing. As the current demand increases, 317 and 337 ICs start heating up and ICs take more drop across them which reduce the output voltages. As from the above practical experience, power dissipation in both the ICs is found greater than their internal tolerable limits. So it is recommended to use heat sinks to aid cooling the ICs and to increase the lifespan of these voltage regulator ICs.
The power supply circuit designed in this project can be used as power adaptor for electronic devices and can be used with chipsets that need a negative power supply. The circuit can be used to power electronic components like OP-AMPS, Bipolar amplifiers, and constant current regulators.